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Timing re-profiled cams, etc |
Part One
Dealing with re-profiled cams, whether reground or billet, puts you into uncharted territory. Cam grinders will provide a cam card with some kind of specs for the cam. These usually include duration and timing with some specified checking clearance often 0.050”. This doesn’t actually give you any real idea of where to position the cam at TDC for the timing cylinder. To find a real reference for timing cams I’ve found the most reliable method is using lobe centers. On twin cam engines this also has the benefit of giving you a reference to adjust torque peak/RPM up or down for your application.Say that the advertised timing is 32 BTDC opening and 62 ABDC closing for intake at 0.050” checking clearance, a duration of 274 degrees. The lobe center would then be at 137 degrees of total. Minus the 32 degrees BTDC this is 105 degrees ATDC. Although the timing at running clearance will be considerably greater the lobe center will not change. So the trick here is to set the cam up so that it’s on its lobe center, and the crank is on 105 degrees ATDC.
Similarly the exhaust lobe center is found. Assuming a 62BBDC opening and 32ATDC closing at 0.050” checking clearance or 274 degrees duration gives a lobe center 137 degrees of total, minus 62 BBDC degrees, putting the lobe center at 75 degrees after BDC.
Finding the lobe center for the cam is similar to finding true TDC. Using a dial indicator reading the lifter rotate the cam until the point of highest lift is found. Zero the indicator and then work to a point some 0.020” before and after the high point and divide to find the true center of the lobe.
All of this sounds pretty daunting. Indeed, installing reprofiled cams does require great care and attention to detail. Not just in getting the timing right, but also insuring sufficient clearance over the piston. This will mean fitting and refitting the cylinder head several times.
Begin by installing the valves without seals and very light springs (I use carb damper springs cut in half) in the front cylinder only. Put the cams in roughly the same orientation as stock cams would be with the clearance set up at 0.020”. Make sure you can spin the lifters in their bores so you’re sure the valves are closed.
Set up your degree wheel and find TDC as explained at faqcams.html. Offer up the head without a gasket and torque it down. Install the cam wheels, tension the upper chain and install the cam wheel bolts. Rotate the engine gently through two full revolutions to make sure there is no interference in the valve gear. Stop at the slightest hint of collision twixt valve and piston.
Having returned to TDC without a problem set up a dial gauge on the exhaust lifter using a tip that is long and thin enough to not foul the cam lobe as it turns. Preload the gauge to at least 0.600” to be sure that you don’t run out of travel as the valve is depressed. Now turn the engine in its normal direction. For reference note the point on the degree wheel where the valve begins to move. Continue turning until the valves begins to close this highest reading is lobe lift and valve lift. Move the crank back and forth several times until you are sure of the highest reading.
Now turn the crank backwards until you are some 0.060” short of full lift. Turn the crank forward until the gauge reads 0.020” less than full lift and note the degree reading. Continue forward until you are again at 0.020” less than full lift and note this reading as well. You are probably getting used to this by now. Halfway between these two degree readings is lobe center. Compare this to your calculated lobe center (75 degrees ABDC in our example) they probably will only be close. Move the crank backwards past 75 Degrees and come forward until you are dead on.
Remove the cam wheel bolts and free up the center so that you can rotate the cam. The piston is now well down the bore so interference won’t be a problem since you only have two valves to worry about. Were all of the valves in the head the risk of collision in other cylinders would be too great. Using the dial gauge and the same procedure 0.020” before make a mark on the thrust flange and the front cam cap. Go to 0.020” after full lift and make a second mark on the cap. Set the cam halfway between these marks and loosen the second cam cap and slip a strip of business card under the bearing and snug the cap down. This is just a safeguard.
You may now refit the center and one bolt. Remove the strip of card. Bring the crank around to TDC and fit the other bolt. Turn the engine through again, noting when the valve begins to open, verify that lobe center is truly correct and note where the valve closes. Use the marks you made on the thrust face of the cam and the front cap for lobe center. These marks need not be very deep as you're going to make more substantial ones for TDC. Move the dial set up to the intake cam and repeat the procedure exactly until that cam is also set to lobe center and you have noted open and close degrees.
Return once more to TDC and mark the front cam cap and thrust face of both cams with an index. A sharp chisel held to mark the cam and cap at once is sufficient. Remember to clean up any burrs left by this process. You’ll have time the head will come off and go on at least twice more before you’re finished.
Part Two
Re-profiled cams: the next step
First a quick review: you’ve now set up the cams on their lobe centers. After returning to TDC you’ve made a timing reference mark on the thrust face of the cam and the front cam cap of each cam. Now you can pull the head again and install the valves with their seals, spring seats and retainers, and set the clearances to the cam grinder's specs.
Let me digress a moment. Regrinding stock cams means the base circle of the lobe must be reduced to achieve the higher lift and new profile. This leaves you with a problem in the clearance setting. Suddenly you’re into extra thick shims. At about $8 a pop you’ve added almost $100 to the price of your endeavor - not to mention fiddling around and trying to successfully measure 0.100” clearances with feeler gauges. My approach is to compare the base circle of the reground cams to the stock base circle. Say for illustration the new base circle is 0.080” smaller than stock. This means that the assembled height of the valve must be 0.080” higher than before to come back into the range of stock valve shims.
So if you were to measure the distance from the gasket face for the valve covers to the stem of the valve before tearing down the head and record this you would reduce this by 0.080” in the course of the valve job. Stock cams will come to about the middle of the shim range if the stems are at 0.835” below the VC gasket face. There is in fact considerable variation in stock Jag cams. Fortunately our machine shop records the stem heights and sends me back a finished head within a few thousandths of that figure.
In the course of a good five angle valve job where the seats are not replaced most of the distance lost to cam grinding will be cut away. Finishing up with a bowl hog will improve flow considerably short of serious port work. The real problem here is that eventually the valve becomes pocketed by the lowering of the seat. Reground cams that don’t go for exaggerated lifts won’t cause this problem, but if you’re looking for high lift and long duration, billet cams are the only way to go. I’ve heard of welded hard faced and reground cams, but don’t have any experience with them. All cam grinders Parkerize or otherwise re-harden their lobes. Well maybe I shouldn’t say all. All reputable cam grinders do. Crane, Isky and Crower are three I’ve had experience with.
Back to the timing of the cams. The portion of the valve opening that is changed with extended duration is first the overlap when both valves are open and the closing which occurs later when the piston is moving opposite the obvious. Intakes closing after BDC and exhaust closing BTDC.
First the overlap portion: when the piston is still moving up, when the intake valve opens it would seem that the charge would be forced back into the intake tract. This is somewhat true, hence the nasty, stuck choke idle of hot cams - the trade off! Two things however prevent this from happening off idle. First is column inertia. Second is ram tuning: the intake charge is trying to get to the cylinder and the inertia of the charge will keep it moving into the cylinder even though the negligible push of the piston at the end of its upward travel is trying to push it back. Remember we are in overlap so column inertia is still scavenging the cylinder.
Second we have the ram effect: when the intake valve opens there is a negative pressure wave sent up the intake tract towards the plenum or the air cleaner. When the wave reaches this point it is reflects back down the tract as a positive wave and at certain RPM this wave arrives at the intake valve as a positive wave and actually pushes the charge into the cylinder in spite of the rising piston.
Conversely, the exhaust valves opening sends a positive wave down the tract that flips over at the collector and returns as a negative wave that arrives at the back of the valve just as it is opening at certain RPM. Plus this column inertia of the moving gases and the cross scavenging caused by the gases from the other cylinders on the same collector. All of which overcome the apparent conundrum of a valve opening whilst he piston is moving in the wrong direction. Thence the bundle of snakes exhaust headers found on balls out engines, as well as the plenum and runners seen on practically every engine in the modern fold of fuel injection engines.
Bob Sharp’s Datsun engines in SCCA racing actually used a long and short inlet trumpet on every other barrel of the Webers and spread their RPM range out a little. Headers with un-equal length runners also spread the ram effects across the rev range making for a bit more tractability.
But we’re talking about timing of cams here. What we get to is lobe centers and inter-lobe angles. From our original example, we have an intake lobe center of 105 degrees ATDC or 75 degrees BBDC. Likewise the exhaust LC is 75 degrees ABDC for a total of 150 degrees between lobe centers. Increasing the interlobe angle will move the torque peak up the RPM scale. Since HP is a unit time function, moving the peak torque up the scale will increase the HP. A look at the various variable valve timing schemes, excepting Honda’s VTEC, will show that the general strategy is to retard the exhaust cam timing. More interlobe angle, higher RPM at max. torque, more HP.
Having digressed for about three miles, lets put the valves back into the head and set the valve clearances. Next we’ll get into claying the pistons to measure minimum valve to piston clearance.
Once you’ve set the clearances on one cam, set up your dial gauge on the #1 lifter and measure the depression of the valve when the front cylinder is on TDC. Record this - it will help you later when you're trying to tune for your set up. It’s always good to have a whole bunch of references so you can cross check.
A last note for this chapter. The two bolt, full circle cam wheel centers have 136 teeth. This works out to be 2.6 degrees per tooth of cam timing, or 5 degrees crankshaft. In reality the cam centres are somewhat vernier so rotating the cam centre 180 degrees may help you split the difference. However, the bottom line is that we are not splitting hairs here. These are Jaguar engines. Until you get into big dollar stuff very fine adjustments ain’t happening.
Part Three
Assembling the head: a special note if you are using valve stem seals. There are two type of valve retainers and matching keepers. Engines without seals used the longer of the two. If you are installing stem seals you must use the short retainers and matching keepers. You should also use a seal that is a little more substantial than the stock products. Some people like the teflon seals from Perfect Circle. I feel that these tend to run a bit drier than is healthy for the guides. There are several metal and high density rubber seals with springs available and I would choose on of these - if for no other reason than they are more forgiving to install. You will also need to use XJ6 spring seat with seals. See retro-fitting valve seals elsewhere in this FAQ section.
Assemble a test valve by installing the spring seat and seal, the valve and your light spring again along with a 0.110” shim and the tappet. Measure the distance to the tappet from the gasket surface. Now depress the valve as far as it will go and measure the new distance. The difference between the two measurements must be at least 0.060” greater than the lobe lift you measured before. Actually you can take this figure from the cam card. In fact you should get together with the machine shop before the valve job is begun to achieve this clearance. You should also have them test your valve springs to make sure that they will not become coil bound with your new lift.
If this has been satisfactorily taken care of you may now assemble the head and make your valve adjustment. Remember to only adjust one cam at a time and completely release the other cam to prevent the valves hitting one another. I put 3/8 x ¾ bolts with a nut into the cam bolt holes and snug the nut down to prevent the bolts protruding past the back and fouling the head. Take a couple of pieces of 2x4 protected with rags to set the ends of the head on during the adjustment, head stands are prone to tipping over on you while you’re turning the cam. Turn the cams with a screw driver using the bolts. Be careful not to ding the gasket surface.
The adjustment process is outlined in the Jaguar manual. Install the cam and torque the caps first using only a flat washer. Now turn the cam through at least three full turns to settle things. Here’s a tidy way to do the clearances. Measure the clearance with feeler gauges and record the figure on the gasket surface below the valve with a felt tip pen.
When all six have been recorded, remove the cam. Take out a single lifter and mic the shim. Say you measured the clearance at 0.017 and the cam card says to set for 0.010 you will need a shim 0.007 thicker.
Select the proper shim and set it in place. Now erase your old measurement first and then install the tappet. When you’ve done all six reinstall the cam and begin measuring again remembering to turn the cam through again first. If the clearance is correct make a slash mark on the gasket surface above the valve. Don’t be surprised if some of the valves aren’t right. Go back and make the final adjustments as required. Once you have a row of six slashes along the upper gasket surface, go back and measure each valve again and if correct turn the slashes into X’s.
Once each cam has a row of X’s along the upper gasket face, install one cam now using a lock and flat washer on the studs and bring that cam up to the TDC marks you made when finding lobe centers. Loosen the second cam cap and using the strip of card method lock this cam down.
Installing the second cam requires great care in making sure that it comes home dead on the TDC marks. Remember you now have greater lift and overlap than stock and the chances of bending a valve are considerably greater as a result. Lock this cam down with a strip of card as well and remove the bolts you were using to turn the cams.
A couple of last words/tips. Never install anything but new tappets with a new cam. Never move a used lifter to a different location if you’re doing a valve job. Always use great gobs of cam lube on the tappets. Never do a valve job or anything else to a cylinder head without installing a tappet guide retainer kit.
Like retainers when Jaguar started using seals they also made short tappets. It can be argued that longer tappets will deflect less resulting in less wear. I’ve never noticed any increased wear in the short tappet engines. It cannot be argued that the longer tappets aren’t heavier.
Next time we’ll install the head and clay the pistons for clearance.
Part Four
Claymation: for this next bit you’ll need some modeling clay. Roll out thirty snakes about 5/16 dia. by 4 ½ long. Lay these across the top of each piston transversely, one in the center and two each fore and aft. Trim them so they can ride the piston without dragging the cylinder walls.
Now, with the front cylinder On TDC, offer up the head once more without a gasket and torque it down. Install the cam wheels, tension the upper chain and install one bolt in each cam wheel. Remove the card strips. Turn the engine and install the second cam bolt. Turn the engine through until the rear cylinder is at TDC on compression. Remove one cam bolt each side and bring the engine to TDC front cylinder compression. Reinstall the card strips to lock the cams, and remove the last bolt from each. Then remove the head again.
You will now have a distinct impression of the valves in the clay, and perhaps some impression from the head as well. Carefully measure the compressed portions of the clay. This will give you the working clearance over the valves and head. Any thing less than 0.060” is not acceptable. You may be able to achieve an acceptable clearance by advancing the exhaust, or retarding the intake cams, but unless you have very high compression pistons, or lobe lifts above 0.450”, you should find your clearances acceptable.
Assuming that this all worked out you are ready to put the head on for the last time. Clean things up, and slather on a bit of cam lube. You’ll be putting more on as you install the cam bolts.
If this is a 3.8 engine I would forego the stamped steel head gasket and obtain a composition gasket. We can supply you with this.
Once the engine is assembled I strongly advise prelubing . Obtain an XJ6 oil pressure warning light switch adapter. This screws into the oil gallery in place of one of the 6 plugs. From your home store buy a one or two gallon garden sprayer (the kind with a pump handle and a hose coming off of the top). Remove the sprayer from the hose. Buy a 1/8" pipe to hose adapter and install it in the oil pressure light adapter then install it in place of a gallery plug. Attach the hose to it with a hose clamp. Add a gallon of oil to the sprayer and pump it up. Temporarily loosen the banjo bolts at the back of the head until you are sure oil is reaching there. Then keep an eye on the oil level in the sprayer so you don’t pump air into the system. When it’s close to empty release the pressure by removing the pump.
I think that about covers this particular subject!
Only the Fool On The Hill would claim to know everything there is to know about any given subject - including Jaguar engines. I am only 40 years into my apprenticeship, but if you think I might be able to help you in any way, don't hesitate to contact me at Classic Jaguar.Have fun!
Lee Merrifield
Master Mechanic
Team CJIf you have any questions relating to the above described procedures, or any other technical issues, feel free to contact me at techsupport@classicjaguar.com